Collect blood samples from patients belonging to multiple-case families with breast cancer. Ascertain new patients with breast cancer from among the M.D. Anderson tumor population who have a family history of the neoplasm, providing source of additional samples. Collect blood samples from female members of control families alredy on file and continually being ascertained and studied by this laboratory. Process the blood samples for the ABO, Rh, MNS, P, Kell, Duffy blood group systems; haptoglobin serum protein; phosphoglucomutase 1 and 2 and acid phosphatase red blood cell enzyme system. Prepare leukocyte cultures and process for routine karyotype analyses and G- and C-band variants. Compare frequency of protein markers and chromosome abnormalities and/or variants in patients from the different risk groups with matched controls. Relate the occurrences of protein markers and chromosome abnormalities and/or variants in the same subjects, whether patient or control.